Electrospray - Wikipedia, The Free Encyclopedia

7/29/2019 Electrospray - Wikipedia, The Free Encyclopedia

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ElectrosprayFrom Wikipedia, the free encyclopedia

The name electrospray is used for a device that employs electricity to disperse a liquid or for the fine aerosol

resulting from this process. The method is sometimes improperly called electrohydrodynamic atomization. High

voltage is applied to a liquid supplied through an emitter (usually a glass or metallic capillary). Ideally the liquid

reaching the emitter tip forms a Taylor cone, which emits a liquid jet through its apex. Varicose waves on the

surface of the jet lead to the formation of small and highly charged liquid droplets, which are radially dispersed

due to Coulomb repulsion.

An electrospray device

taken at the Nottingham

University.

A close-up of an

electrospray device

taken at the Nottingham

University, the jet of

ionised spray is visible

within the image.

Contents

1 History

2 Mechanism

2.1 Effect of small electric fields on liquid menisci

2.2 The Taylor cone

2.3 Singularity development

2.4 Closing the electrical circuit

3 Applications

3.1 Electrospray ionization

3.2 Electrospinning

3.3 Colloid thrusters

3.4 Deposition of particles for nanostructures

3.5 Air purifiers

3.6 Liquid Metal Ion Sourcing

4 References

History
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In the late 16th century William Gilbert[1] set out to describe the behaviour of magnetic and electrostatic

phenomena. He observed that, in the presence of a charged piece of amber, a drop of water deformed into a

cone. This effect is clearly related to electrosprays, even though Gilbert did not record any observation related

to liquid dispersion under the effect of the electric field.

In 1882, Lord Rayleigh theoretically estimated the maximum amount of charge a liquid droplet could carry;[2]

this is now known as the "Rayleigh limit". His prediction that a droplet reaching this limit would throw out fine

ets of liquid was confirmed experimentally more than 100 years later.[3]

In 1914, John Zeleny published work on the behaviour of fluid droplets at the end of glass capillaries. [4] This

report presents experimental evidence for several electrospray operating regimes (dripping, burst, pulsating, and

cone-jet). A few years later, Zeleny captured the first time-lapse images of the dynamic liquid meniscus. [5]

Between 1964 and 1969 Sir Geoffrey Ingram Taylor produced the theoretical underpinning of

electrospraying.[6][7][8] Taylor modeled the shape of the cone formed by the fluid droplet under the effect of an

electric field; this characteristic droplet shape is now known as the Taylor cone. He further worked with J. R.

Melcher to develop the "leaky dielectric model" for conducting fluids.[9]

Mechanism

To simplify the discussion, the following paragraphs will address the case of a positive electrospray with the high

voltage applied to a metallic emitter. A classical electrospray setup is considered, with the emitter situated at a

distance from a grounded counter-electrode. The liquid being sprayed is characterized by its viscosity ,

surface tension , conductivity , and relative permittivity .

Effect of small electric fields on liquid menisci

Under the effect of surface tension, the liquid meniscus assumes a semi-spherical shape at the tip of the emitter.

Application of the positive voltage will induce the electric field:[10]

where is the liquid radius of curvature. This field leads to liquid polarization: the negative/positive charge

carriers migrate toward/away from the electrode where the voltage is applied. At voltages below a certain

threshold, the liquid quickly reaches a new equilibrium geometry with a smaller radius of curvature.

The Taylor cone

Voltages above the threshold draw the liquid into a cone. Sir Geoffrey Ingram Taylor described the theoretical

shape of this cone based on the assumptions that (1) the surface of the cone is an equipotential surface and (2)

the cone exists in a steady state equilibrium.[6] To meet both of these criteria the electric field must have

azimuthal symmetry and have dependence to balance the surface tension and produce the cone. The

solution to this problem is:
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where (equipotential surface) exists at a value of (regardless of R) producing an equipotential

cone. The magic angle necessary for for all R is a zero of the Legendre polynomial of order 1/2,

. There is only one zero between 0 and at 130.7099, which is the complement of the

Taylor's now famous 49.3 angle.

Singularity development

The apex of the conical meniscus cannot become infinitelly small. A singularity develops when the hydrodynamic

relaxation time becomes larger than the charge relaxation time .[11] The undefined

symbols stand for characteristic length and vacuum permittivity . Due to intrinsic varicose instability, the

charged liquid jet ejected through the cone apex breaks into small charged droplets, which are radially dispersed

by the space-charge.

Closing the electrical circuit

The charged liquid is ejected through the cone apex and captured on the counter electrode as charged dropletsor positive ions. To balance the charge loss, the excess negative charge is neutralized electrochemically at the

emitter. Imbalances between the amount of charge generated electrochemically and the amount of charge lost at

the cone apex can lead to several electrospray operating regimes. For cone-jet electrosprays, the potential at

the metal/liquid interface self-regulates to generate the same amount of charge as that lost through the cone

apex.[12]

Applications

Electrospray ionization

see also the main article on Electrospray ionization

Electrospray became widely used as ionization source for mass spectrometry after the Fenn group successfully

demonstrated its use as ion source for the analysis of large biomolecules.[13]

Electrospinning

see also the main article on Electrospinning

Similarly to the standard electrospray, the application of high voltage to a polymer solution can result in the

formation of a cone-jet geometry. If the jet turns into very fine fibers instead of breaking into small droplets, the

process is known as electrospinning .

Colloid thrusters

see also the main article on Colloid thrusters

Electrospray techniques are used to control satellites, since the fine-controllable particle ejection allows precise

and effective thrusts.

Deposition of particles for nanostructures
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Electrospray may be used in nanotechnology,[14] for example to deposit single particles on surfaces. This is

done by spraying colloids on average containing only one particle per droplet. The solvent evaporates, leaving

an aerosol stream of single particles of the desired type. The ionizing property of the process is not crucial for

the application but may be used in electrostatic precipitation of the particles.

Air purifiers

see also the main article on Air purifiers

Particulates suspended in air can be charged by the aerosol generated by an electrospray, manipulated by an

electric field and collected on a grounded electrode. This approach minimizes the production of ozone which is

common to other types of air purifiers.

Liquid Metal Ion Sourcing

see also the main article on Liquid metal ion source

Liquid metals can be used to create ion sources for ion implantation techniques and focused ion beaminstruments.

References

1. ^ Gilbert, W. (1628) De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet

and Magnetic Bodies, and on That Great Magnet the Earth), London, Peter Short

2. ^ Rayleigh, L. (1882). "On the Equilibrium of Liquid Conducting Masses charged with Electricity".

Philosophical Magazine14: 184186.

3. ^ Gomez, A & Tang, K (1994). "Charge and fission of droplets in electrostatic sprays.". Physics of Fluids6(1): 404414. Bibcode 1994PhFl....6..404G (http://adsabs.harvard.edu/abs/1994PhFl....6..404G) .

doi:10.1063/1.868037 (http://dx.doi.org/10.1063%2F1.868037) .

4. ^ Zeleny, J. (1914). "The electrical discharge from liquid points, and a hydrostatic method of measuring the

electric intensity at their surfaces.". Physical Review3 (2): 69. Bibcode 1914PhRv....3...69Z

(http://adsabs.harvard.edu/abs/1914PhRv....3...69Z) . doi:10.1103/PhysRev.3.69

(http://dx.doi.org/10.1103%2FPhysRev.3.69) .

5. ^ Zeleny, J. (1917). "Instability of electrified liquid surfaces.". Physical Review10 (1): 16. Bibcode

1917PhRv...10....1Z (http://adsabs.harvard.edu/abs/1917PhRv...10....1Z) . doi:10.1103/PhysRev.10.1


6. ^ ab Sir Geoffrey Taylor (1964). "Disintegration of Water Droplets in an Electric Field". Proc. Roy. Soc.

London. Ser. A280 (1382): 383. Bibcode 1964RSPSA.280..383T(http://adsabs.harvard.edu/abs/1964RSPSA.280..383T) . doi:10.1098/rspa.1964.0151

(http://dx.doi.org/10.1098%2Frspa.1964.0151) . JSTOR 2415876 (http://www.jstor.org/stable/2415876) .

7. ^ Taylor, G. (1965) The force exerted by an electric field on a long cylindrical conductor. Proceedings of the

Royal Society of London A: Mathematical, Physical & Engineering Sciences, 291, 145-158

8. ^ Taylor, G. (1969) Electrically Driven Jets. Proceedings of the Royal Society of London A: Mathematical,

Physical & Engineering Sciences, 313, 453-475

9. ^ Melcher, J. R. & Taylor, G. (1969) Electrohydrodynamics: A Review of the Role of Interfacial Shear

Stresses. Annual Review of Fluid Mechanics, 1, 111-146

10. ^ L. B. Loeb, A. F. Kip, G. G. Hudson, W. H. Bennett (1941). "Pulses in negative point-to-plane corona".

Physical Review60 (10): 714722. Bibcode 1941PhRv...60..714L

(http://adsabs.harvard.edu/abs/1941PhRv...60..714L) . doi:10.1103/PhysRev.60.714


11. ^ Fernndez de la Mora, J.; Loscertales, I. G. (1994). "The current emitted by highly conductive Taylor

cones.".Journal of Fluid Mechanics260: 155184. Bibcode 1994JFM...260..155D

(http://adsabs.harvard.edu/abs/1994JFM...260..155D) . doi:10.1017/S0022112094003472
http://dx.doi.org/10.1017%2FS0022112094003472http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1994JFM...260..155Dhttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Journal_of_Fluid_Mechanicshttp://en.wikipedia.org/wiki/Electrospray#cite_ref-11http://dx.doi.org/10.1103%2FPhysRev.60.714http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1941PhRv...60..714Lhttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Physical_Reviewhttp://en.wikipedia.org/wiki/Electrospray#cite_ref-10http://en.wikipedia.org/wiki/Electrospray#cite_ref-MelcherTaylor_9-0http://en.wikipedia.org/wiki/Electrospray#cite_ref-Taylor1969_8-0http://en.wikipedia.org/wiki/Electrospray#cite_ref-Taylor1965_7-0http://www.jstor.org/stable/2415876http://en.wikipedia.org/wiki/JSTORhttp://dx.doi.org/10.1098%2Frspa.1964.0151http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1964RSPSA.280..383Thttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Proceedings_of_the_Royal_Society_A#Proceedings_of_the_Royal_Society_Ahttp://en.wikipedia.org/wiki/Electrospray#cite_ref-Taylor1964_6-1http://en.wikipedia.org/wiki/Electrospray#cite_ref-Taylor1964_6-0http://dx.doi.org/10.1103%2FPhysRev.10.1http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1917PhRv...10....1Zhttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Physical_Reviewhttp://en.wikipedia.org/wiki/Electrospray#cite_ref-5http://dx.doi.org/10.1103%2FPhysRev.3.69http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1914PhRv....3...69Zhttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Physical_Reviewhttp://en.wikipedia.org/wiki/Electrospray#cite_ref-4http://dx.doi.org/10.1063%2F1.868037http://en.wikipedia.org/wiki/Digital_object_identifierhttp://adsabs.harvard.edu/abs/1994PhFl....6..404Ghttp://en.wikipedia.org/wiki/Bibcodehttp://en.wikipedia.org/wiki/Physics_of_Fluidshttp://en.wikipedia.org/wiki/Electrospray#cite_ref-3http://en.wikipedia.org/wiki/Philosophical_Magazinehttp://en.wikipedia.org/wiki/Electrospray#cite_ref-2http://en.wikipedia.org/wiki/Electrospray#cite_ref-gilbert_1-0http://en.wikipedia.org/wiki/Liquid_metal_ion_sourcehttp://en.wikipedia.org/wiki/Ozonehttp://en.wikipedia.org/wiki/Air_purifierhttp://en.wikipedia.org/wiki/Electrostatic_precipitatorhttp://en.wikipedia.org/wiki/Aerosolhttp://en.wikipedia.org/wiki/Colloidshttp://en.wikipedia.org/wiki/Electrospray#cite_note-14http://en.wikipedia.org/wiki/Nanotechnology


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(http://dx.doi.org/10.1017%2FS0022112094003472) .

12. ^ Van Berkel, G. J.; Zhou, F. M. (1995). "Characterization of an electrospray ion source as a controlled-current

electrolytic cell".Analytical Chemistry67 (17): 29162923. doi:10.1021/ac00113a028

(http://dx.doi.org/10.1021%2Fac00113a028) .

13. ^ Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. (2007). "Electrospray ionization for

mass spectrometry of large biomolecules.". Science246 (4926): 6471. Bibcode 1989Sci...246...64F

(http://adsabs.harvard.edu/abs/1989Sci...246...64F) . doi:10.1126/science.2675315

(http://dx.doi.org/10.1126%2Fscience.2675315) . PMID 2675315 (//www.ncbi.nlm.nih.gov/pubmed/2675315)

.14. ^ Salata, O.V. (2005). "Tools of nanotechnology: Electrospray". Current Nanoscience1: 2533. Bibcode

2005CNan....1...25S (http://adsabs.harvard.edu/abs/2005CNan....1...25S) . doi:10.2174/1573413052953192

(http://dx.doi.org/10.2174%2F1573413052953192) .

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